1. Field of the Invention
The invention is related to the field of sensors, and in particular, to sensors that detect frozen and non-frozen liquid.
2. Statement of the Problem
Sensors that measure water and ice are well known. Special versions of these sensors have been developed for external airplane mounting. These special sensors collect data on ice and water concentrations in the atmosphere while the airplane is in flight.
Water Sensor
FIG. 1 illustrates water sensor 100 in an example of the prior art. Water sensor 100 is attached to an airplane, and as the airplane flies, sensor 100 detects non-frozen water in the atmosphere. Water sensor 100 includes a cylinder that has a temperature sensor and that is connected to two supports. The two supports are attached to the airplane. Water sensor 100 also includes circuitry and associated power and temperature wiring.
The temperature wiring runs from the temperature sensor to the circuitry. The power wiring runs from the circuitry through one support and the cylinder, and then back down the other support to a ground. Although not shown for clarity, the power wiring is typically wound around a core within the cylinder, and a thermal-conducting material surrounds the power wiring to form the exterior of the cylinder. Thus, the power wiring is able to heat the cylinder based on the power supplied by the circuitry.
The circuitry controls the power transferred over the power wiring to maintain the cylinder at a constant temperature. To perform this task, the circuitry receives a signal indicating the temperature of the cylinder from the temperature sensor over the temperature wiring. In response to the cylinder temperature, the circuitry adjusts the power transferred over the power wiring to maintain the constant temperature. Thus, if the cylinder temperature drops below the constant temperature, then the power is increased, but if the cylinder temperature rises above the constant temperature, then the power is decreased.
When water strikes the surface of the cylinder, the water spreads over the cylinder surface to provide a cooling effect. To maintain the constant temperature, more power is required to evaporate the water. Ice tends to bounce off of the cylinder, so the ice does not cause a similar increase in power. Thus, the amount of power consumed by the cylinder correlates to the concentration of water in the atmosphere through which the airplane flies.
In addition to water, wind also affects the power consumed by the cylinder. Wind that strikes the cylinder also provides a cooling effect, so as the wind increases, the power required to maintain the constant temperature also rises. As the wind striking the cylinder decreases, then the power required to maintain the constant temperature also decreases. The airspeed of the airplane is the dominant factor in generating the wind that strikes the cylinder.
The circuitry receives an indication of the air speed of the airplane, so the circuitry can allocate the appropriate amount of power consumption to wind, and thus, allocate the appropriate amount of power consumption to the water that strikes the cylinder. Once the power consumption due to the water is determined, the circuitry can determine the concentration of water in the atmosphere.
Although ice tends to bounce off of the cylinder, some ice may collect on the cylinder during flight. In an ice-only precipitation event, sensor 100 can detect this ice due to the power increase required to melt and evaporate the collected ice. The power increase would not correlate well to the concentration of water in the atmosphere because the cylinder's collection efficiency for ice is so poor. In a mixed ice-water precipitation event, the collected ice only serves to generate error in the water precipitation rate calculation.
Water sensor 100 has exhibited problems in practice. Water sensor 100 cannot provide any accurate data regarding ice. This problem is especially acute with respect to clouds, since small ice particles are prevalent in clouds and are of interest to pilots and scientists alike. Water sensor 100 has also proven to be fragile when mounted on an airplane. In addition, water sensor 100 has two supports that require separate attachments to the airplane. Although some attachment to the airplane is required, multiple attachments are not desirable when considering the structural integrity of the airplane.
Total Ice and Water Sensor
FIG. 2 illustrates ice/water sensor 200 in another example of the prior art. Ice/water sensor 200 is also attached to an airplane, and as the airplane flies, ice/water sensor 200 detects the combined ice and water in the atmosphere. Ice/water sensor 200 includes a cylinder that has a temperature sensor and that is connected to a support. The support is attached to the airplane and allows the cylinder to rotate. A vane on the cylinder, along with the pivoting support, causes one end of the cylinder to point into the wind. The cylinder has a reentrant shape at the end that points into the wind. The reentrant shape forms an inverted cone that extends into the end of the cylinder that points into the wind. As the airplane flies, ice and water that enter the cone are trapped within the cone.
Ice/water sensor 200 also includes circuitry and associated power and temperature wiring. The temperature wiring runs from the temperature sensor to the circuitry. The power wiring runs from the circuitry through the support and the cylinder, and then back down the support to a ground. Thus, the power wire is able to heat the cylinder based on the power supplied by the circuitry.
The circuitry and power wiring maintain the cylinder at a constant temperature. To perform this task, the circuitry receives a signal indicating the temperature of the cylinder from the temperature sensor over the temperature wiring. In response to the cylinder temperature, the circuitry adjusts the power transferred over the power wiring to maintain the constant temperature. Thus, if the cylinder temperature drops below the constant temperature, then the power is increased, but if the cylinder temperature rises above the constant temperature, then the power is decreased.
Ice and water that are trapped in the cone provide a cooling effect, so more power is required to melt the ice and evaporate the melted ice and water to maintain the constant temperature. Thus, the amount of power consumed by the cylinder correlates to the concentration of both ice and water in the atmosphere through which the airplane flies.
In addition to water, wind also affects the power consumed by the cylinder. As the wind striking the cylinder increases, then the power required to maintain the constant temperature also rises. As the wind striking the cylinder decreases, then the power required to maintain the constant temperature also decreases. The airspeed of the airplane is the dominant factor in generating the wind that strikes the cylinder.
The circuitry receives an indication of the air speed of the airplane, so the control circuitry can allocate the appropriate amount of power consumption to wind, and thus, allocate the appropriate amount of power consumption to the ice and water that strike the cylinder. Once the power-consumption due to ice and water is determined, the circuitry can determine the combined concentration of ice and water in the atmosphere.
Ice/water sensor 200 has exhibited problems in practice. Ice/water sensor 200 cannot provide any accurate data regarding only ice, or regarding only wafer. This problem is especially acute in clouds where small particles of ice are prevalent, and are of interest to pilots and scientists alike. Ice/water sensor 200 has also proven to be fragile when mounted on an airplane. In addition, the aerodynamics caused by pointing the cylinder into the wind causes very small particles of ice and water that should fly into the cone to fly around the cone instead. This loss of small particles adds inaccuracy to the results, especially when a cloud is encountered that contains a large concentration of these smaller ice particles.
Ice/Water Sensor System
Sensors 100 and 200 can be used together to obtain data specific to the ice concentrations in the atmosphere. Ice/water sensor 200 is used to get the combined ice/water concentration, and water sensor 100 is used to get the water-only concentration. The water-only concentration is subtracted from the combined ice/water concentration to obtain the ice-only concentration.
Unfortunately, the combined use of sensors 100 and 200 to determine the ice-only concentration is prone to error, because sensors 100 and 200 have different collection efficiencies due to their different shape and size. The collection efficiency is a ratio of the total amount of water in a given volume versus the actual amount of water that is collected and evaporated by the sensor traveling through the volume. For example, water sensor 100 may collect and evaporate 95% of the water in a given volume, but ice water sensor 200 may only collect and evaporate only 80% of the water in the same volume. When the water concentration is subtracted from the ice/water concentration, the two water concentrations are not equivalent due to the different collection efficiencies of sensors 100 and 200. The resulting ice concentration is inaccurate because the two water concentrations were not equivalent.
In addition, ice/water sensor 200 does not collect some smaller ice particles, which further skews the result. The combined use of sensors 100 and 200 also leads to an undesirable number of attachments to the airplane. The combined use of sensors 100 and 200 does not provide a robust system that can stand up to the rigors of external airplane mounting.